1. Field of the Invention
This invention relates to ac power switching devices having operating coils powered by ac current, and more particularly to such devices in which power to the operating coil is controlled by logic level signals.
2. Background Information
Many electric switching devices such as contactors and motor starters have an operating coil energized by ac power. This ac power may be 120 volts or the line voltage of the circuit being controlled which may be, for example, up to 600 volts or even more. Conventionally, the control circuits for the coils have also operated at 120 volts or at the line voltage. Such control circuits have included several conductors operating at these voltages and extending between the operating coil on the contactor or starter and a control panel. Adequate protection must be provided for all components to which such voltages are applied. In addition, especially in the instances where line voltages above 120 volts are present, workers trained in handling such voltages are required for installation and service of the devices.
Recently, programmable logic controllers (PLC) have been developed for controlling contacts and starters. These controllers utilize logic level signals (typically 0-5 volts) to switch 120 volts and are implemented on printed circuit boards. Hence, lines carrying 120 volts are required between the printed circuit boards and the contactors and starters, and the boards must be designed to handle the power drawn by the contactor or starter coil. These PLCs use pulse width modulated logic signals to control the firing angles of solid state switching devices which gate half cycles of the 120 volt ac current to the coil. For instance, full conduction is used to initiate closure of the contacts of the device and then conduction is scaled back to provide the reduced level of holding current needed once the contacts are closed.
The PLCs provide the capability of controlling the contactors and starters remotely, such as, by a computer which may coordinate the operation of a large number of such devices through a communications network.
Many of the existing contactors and starters, such as, for example, those disclosed in U.S. Pat. Nos. 3,821,671 and 4,309,683 are enclosed within the interior of an insulating housing. The housings have exterior recesses, or cavities, in which auxiliary contact devices are mounted. These auxiliary contacts may be used, for example, to provide an electrical interlock for the device.
There is a need for remotely controlling the large number of power level controlled contactors and starters in service.
There is a further need for remotely controlling such devices without the need for extended lengths of power leads between the devices and control circuits.
There is an additional need for apparatus for easily and economically retrofitting existing devices.